1. Field of the Invention
This invention relates to a navigation system installed in a movable body such as an automobile, and more particularly to a navigation system for a movable body wherein a map as well as a current position, a locus of movement and so forth of an automobile are displayed on a display unit such as a cathode ray tube to provide driving information to a driver.
2. Description of the Prior Art
A navigation system installed in a movable body such as an automobile is generally constituted such that a map of the whole of a country or a plurality of divided regional maps are stored as map information in map storage means and such map information is read out in accordance with a demand of an operator and displayed on a display unit such as a cathode ray tube to provide a map for the driving to the operator.
Further, a current position of a vehicle is found out by a calculation based detection signals from a distance sensor for detecting a travel distance of the vehicle and a direction sensor for detecting a driving direction of the vehicle, and a triangle mark indicative of the current position of the vehicle and a plurality of serial dots indicative of a driving locus are displayed on a map displayed on the display unit so that the position of the vehicle on the map on the screen may be visually discerned. Such navigation system is disclosed, for example, in Japanese Patent Laid-Open No. 53711/1983 and Japanese Patent Publication No. 39800/1984. Another navigation is also known and disclosed, for example, in Japanese Patent Laid-Open No. 165511/1985 which includes means for retrieving presence or absence of another particular map or means (a scroll key) for changing over a map displayed on a screen to another particular map related to such displayed map.
With the conventional systems described above, after driving of a vehicle is started, a road map indicating a current position of the vehicle is retrieved and displayed on the screen. Then, if it is intended to change over the displayed map, an operator will depress one of cursor keys, and in response to the cursor key, another map contiguous to the displayed map of the current position is retrieved. Thus, the next map contiguous to the displayed map in a direction designated by operation of the cursor key is loaded and displayed on the screen.
However, with a system of the type described above, while a map an operator wants to see can be displayed in response to a request by the operator, when the map displayed is changed over, a mark indicative of a current position of a vehicle in which the system is installed is moved together with the map. Accordingly, there is the possibility that, when the initial map is restored, the operator may lose sight of the position of the vehicle and may not grasp the current position on the map. Consequently, when the operator is driving an unknown or unfamiliar location, its way may be lost.